Recovery of organic isocyanates



A nl 28, 1959 A. BLOOM ETAL RECOVERY -OF ORGANIC ISOCYANATES Filed Oct.31, 1956 530t 22:30am :5 2.23m

228502 u uPS Albert Bloom Harlan 8. Frayermuth James B. NorminqtonATTORNEYS United States Patent RECOVERY OF ORGANIC ISOCYANATESApplication October 31, 1956, Serial No. 619,388

10 Claims. (Cl. 202-57) This invention relates to an improvement inprocesses for the production of organic isocyanates and moreparticularly to a process for recovering organic isocyanates from crudemixtures containing the same.

It has long been known that organic isocyanates may "be prepared byreacting phosgene with a primary amine corresponding to the desiredisocyanate. The amine is usually employed in the form of itshydrochloride, although the free amine has been used in some cases. Inplace of phosgene, materials which liberate phosgene during the reactionmay also be employed. It is also known that potassium cyanate may bereacted with an organic sulfate to produce the corresponding organicisocyanate. Another proposed method for producing organic isocyanatesinvolves pyrolyzing an N-substituted carbamate and separating thecorresponding isocyanate from the pyrolysis products before the latterhave had time to reunite and form the starting N-substituted carbamate.The foregoing reactions may be conducted in the absence of an inertorganic liquid diluent or in the presence of any one of a large numberof previously disclosed inert organic liquid diluents.

During these above-described reactions for the production of organicisocyanates, which are often carried out at elevated temperatures, andduring subsequent steps for the treatment of the reaction mass torecover the organic isocyanates therefrom, which usually involves one ormore distillation steps, various by-products and polymers of the desiredorganic isocyanates are produced which results in lower yield of thedesired product. Although many attempts have been made to increase theyield of organic isocyanate and/ or reduce the amount of undesiredby-pr'oducts and residues remaining, no completely satisfactory solutionto the problem has thus far been found. For example, when a reactionmass is produced containing the desired organic isocyanatein an inertorganic liquid diluent, and the said diluent and the major portion ofthe isocyanate are removed by distillation, a substantial amount ofby-product is obtained as a result of polymerization and/ or otherundesirable side reactions during the main reaction and the subsequent.distillation, which by-product both consumes and entraps considerablequantities of the desired organic isocyanate. In US. Patent No.2,680,128, a method is proposed for. more efficient recovery of organicdiisocyanates 'by the addition of a polynuclear hydrocarbon.

plasticizer to the residue containing the diisocyanat'e followed byfractional distillation to recover the diisocyanate content. Thisplasticizer has been found to be often difficult to recover (and reuse)and/or to use in such manner as to attain the desired increased yields.

It is an object of this invention to provide an improved method forrecovering an organic isocyanate from crude mixtures containing thesame. It is another object of this invention to increase the yield ofreactions directed to the production of an organic isocyanat Other b.

k ice jects and advantages will appear as the description proceeds.

The attainment of the above objects is made possible by the instantinvention which comprises a process for recovering the organicisocyanate from 'a crude mixture containing the same comprisingdistilling said isocyanate from said mixture in the presence of a memberof the group consisting of the alkyl, aryl and mixed alkylaryl triestersor orthophosphoric acid which are liquid under the conditions of thedistillation. The process of this invention has been found to enable theattainment of increased yields of the desired organic isocyanate and toenable the recovery of organic isocyanates from residues hithertodiscarded. Although the mechanism by which the instant process achievesthe desired results is not definitely understood, it is believed thatthe above defined liquid phosphate esters employed herein may act (1) toincrease the relative volatility of the organic isocyanate whereby theisocyante is more readily volatilized from the distillation mass, (2) asa chemical exerting a depolymerizing action, (3) as a heat exchangemedium having a depolymerizing action, or (4) as a fluidizing agent forthe by-products or in accordance with any combination of the foregoingmechanisms.

As examples of triesters of ortho-phosphoric acid which may be employedin carrying out the instant invention, there may be mentionedtri-o-cresyl phosphate, tri-m-cresyl phosphate, tri-p-cresyl phosphate,triphenyl-phosphate, diphenyl cresyl phosphate, trirnethyl phosphate,triethyl phosphate, tributyl phosphate, triamyl phosphate, t-ri-(Z-ethylhexyl) phosphate, and mixtures thereof and the like. As stated,they must be liquid under the conditions of the distillation by whichthe isocyanate is removed from the mixture, which distillation may becarried out under atmospheric but preferably under subatmospheric orvacuum conditions and temperatures dependent upon the boiling point ofthe isocyanate under the same conditions. Stated otherwise, thephosphate tri-esters employed herein must be stable, liquid, and have aboiling point higher than that of the organic isocyanate being recoveredunder the particular distillation conditions employed. The amount ofphosphate triester required to achieve the desired results must be suchas to yield a fluid distillation mass, which may be in the form of asolution, dispersion, or suspension or the like. In most cases, however,an amount in excess of this minimum amount is employed since as pointedout above, the phosphate triester may act in accordance with any one ormore of at least four different mechanisms. The optimum amount to beemployed in any particular instance to achieve the desired maximumrecovery of organic isocyanate will be readily determinable by theperson skilled in the art and will depend upon the particular organicisocyanate mixture being treated, the particular phosphate triesteremployed and the conditions of distillation, generally ranging fromabout 25 to 300%, and more preferably about 50 to based on the weight ofthe organic isocyanate in the mixture.

The process of the present invention has been found to be of particularinterest and particularly valuable in the recovery of organicdiisocyanates, particularly as present in a reaction mass resulting fromthe reaction of phosgene with the corresponding aromatic diamine or saltthereof in an inert organic liquid diluent. The instant process isparticularly advantageous in cases where- -in such inert organic diluenthas a boiling point below the instant process is equally applicable tothe recovery of organic isocyanates generally; i.e., aliphatic,cycloaliphatic, alkyl aryl, aralkyl, heterocyclic and aryl mono-, diandpolyisocyanates. Illustrative examples of these organic isocyanateswhich may be recovered in accordance wi h th proc ss of the instinvention, y action of phosgene with the corresponding amine, are hexylisocyanate from hexylarnine, octyl isocyanate from octylamine, dodecyliso-cyanate from dodecylamine, octadecyl isocyanate from octadecylamine,tetramethylene diisocyanate from tetramethylenediamine, pentamethylenediisocyanate from pentamethylenediamine, octamethylene diisocyanate fromoctamethylenediamine, undecamethylene diisocyanate fromundecamethylenediamine, dodecamethylene diisocyanate fromdodecamethylenediamine, 3,3'-diisocyanato dipropylether from 3,3'diarninOdiPropflether, etc., cyclohexyl isocyanate from cyclohexylamine,tetrahydro-a-naphthyl isocyanatc from tetrahydrosot-naphthylamine,tetrahydro-fi-naphthyl isocyanate from tetrahydro-B-naphthylamine, etc.,xylene diiSOcyanates from xylylenediamines, diphenylmethane4,4'-diisocyanate from 4,4'-diamino-diphenylmethane,fi,fi'-diphenylpropane 4,4'-diisocyanate from4,4'.-diaminoafi-diphenylpropane, etc., benzyl isocyanate frombenzylamine, phenylethyl isocyanate from phenylethylamine, p-isocyanatobenzyl isocyanate from paminobenzylamine, etc., phenyl isocyanate fromaniline, p-cetyl phenyl isocyanate from p-cetylaniline, p-dodecylphenylisocyanate from p-dodecylaniline, 5-dodecyl-2- methylphenyl isocyanatefrom 5-dodecyl-o-toluidine, 3- nitro-4-dodecylphenyl-isocyanate from3-nitro-4-dodecyl aniline, p-cetyloxyphenyl isocyanate fromp-cetyloxyaniline, metaphenylene diisocyanate from metaphenylenediamine, p-phenylene diisocyanate from p-phenylenediamine,1-methylphenylene-2,4-diisocyanate from 1- methyl phenylene-2,4-diamine,naphthylene 1,4-diisocyanate from 1,4-naphthylene diamine, 2,6-tolylenediisocyanate from 2,6-toluenediamine, 1,3,5-benzene triisocyanate from1,3,5-benzene triamine, etc., tetrahydrofurfuryl isocyanate fromtetrahydrofurfurylamine, etc. It will also be understood that theinstant process is also applicable to the recovery of organicisocyanates produced by other reactions and from other components forexample by prior art reactions discussed above.

In the preferred embodiment of this invention, the above definedphosphate triester is directly added to the crude liquid reaction massresulting from the reaction of phosgene with the corresponding aromaticamine or salt thereof in an inert organic liquid diluent, and theresulting mixture subjected to distillation to successively separate thesaid inert organic liquid diluent and the organic isocyanate inrelatively increased yields, the residue consisting essentially of thedefined phosphate triester. An additional advantage of this preferredembodiment is that any by-products present in the mixture or producedduring the distillation remain in suspension or solution in the presenceof the phosphate triester and are easily removed from the still in afluid state. However, improved results are also obtained by addition ofthe phosphate triester to the residue of organic isocyanate remainingafter distilling oil the inert organic liquid diluent, or to the residueof by-products and organic isocyanate remaining after distilling offboth the inert organic liquid diluent and the major portion of organicisocyanate. It will be understood, however, that the instant process isapplicable for the recovery of organic isccyanates from other mixturescontaining the same in liquid or solid form.

The distillation process of the instant invention is carried out in wellknown manner, and accordingly need not be described in great detail. Ingeneral, conditions of temperature and pressure (preferablysub-atmospheric or vacuum) employed in the process will be dependentupon the particular organic isocyanates being recovered, the othercomponents of the distillation mass, and the like.

The attached drawing is a flow sheet illustrative of 4 variousembodiments 'of the process of the present invention. As shown, thedesired results maybe obtained by adding suitable amountsjof phosphatetriester to the system at any of the points indicated at arrows A, B orC. From the incoming crude liquid reaction mass containing organicisocyanate, by-products and inert organic liquid diluent there areobtained, as shown, separate fractions containing inert organic liquiddiluent, organic isocyanate, and a residual mixture includingby-products and phosphate triester. It will be understood that any twosuccessive, or all three, distillation steps shown may be carried out inwell known manner in a single continuous distillation column, providedof course that the phosphate triester is added to the system prior tothe final distillation step.

The following examples in which parts are by weight unless otherwiseindicated are illustrative of the instant invention and are not to beregarded as limitative.

Example 1 Into a 2-liter, 4-neck flask is charged a mixture of 609 g.o-dichlorobenzene. This solvent is cooled with stirring to 0-l0 C. and140 g. of phosgene are passed in at this temperature. 122 g. of2,4-tolylene diamine are dissolved in- 203 g. of o-dichlorobenzene byheating to C. This hot solution is then added dropwise to the phosgenesolution at 0-10 C. A fine slurry results. This slurry is stirred andheated to 100-165 C. in about 2% hours while passing phosgene gastherethrough at a rate of approximately 2 g. per minute, at which timesolution is complete. The phosgenation is continued for a shortadditional time, and the solution is then cooled and degassed with drynitrogen. The Weight of the resulting crude solution of 2,4-tolylenediisocyanate in o-dichlorobenzene amounts to 960-980 grams.

Example 2 (For comparative purposes) 240 g. (one quarter) of the aboveresulting crude solution are placed in a 500 cc. round bottom flask andthe o-dichlorobenzene distilled through a 15" glass column having a 4"diameter and packed with glass helicies. The distillation is carried outunder l-2 mm. pressure. The o-dichlorobenzene is recovered at 35 to 40C., and the tolylene diisocyanate at to C. 33 g. of tolylenediisocyanate, analyzing 96% (73% of the theoretical yield) is obtainedWithout the use of. a phosphate triester.

Example 3 30 g. of tri-o-cresyl phosphate are added to 240 g. (onequarter) of the above resulting crude solution of Example 1 and theo-dichlorobenzene and tolylene diisocyanate distilled under vacuum as inExample 2. 38.2 g. of tolylene diisocyanate analyzing 97.6% areobtained, which corresponds to 87.2% of the theoretical yield.

Example 4 240 g. (one quarter) of the crude solution obtained in Example1 are distilled as in Example 2 to the point where the o-dichlorobenzeneis recovered. 30 g. of tri-ocresyl phosphate are added to the still andthe distillation to recover the tolylene diisocyanate carried out as inExample 2. A yield similar to that of Example 3 is obtained.

Example 5 The procedure of Example 2 is repeated, until 32.0 g. oftolylene diisocyanate are obtained, at which time the 12 g. of residueremaining in the flask is still fluid. 15 g. of tri-o-cresyl phosphateare then added to the residue and distillation continuedunder'the sameconditions to recover an additional ca. 6 g. of tolylene diisocyanate ina yield similar to that of Example 3.

In the above examples a mixture of 80% 2,4- and 20% 2,6-tolylenediamines may be employed as initial reactants instead of the 2,4-isomeralone, in which case a corresponding mixture of 2,4- and 2,6-tolylenediisocyanates is obtained.

This invention has been disclosed with respect to certain preferredembodiments, and there will become obvious to persons skilled in the artvarious modifications,

, equivalents or variations thereof which are intended to be includedwithin the spirit and scope of this invention.

We claim:

1. A process for recovering an organic isocyanate from a crude mixturecontaining the same, comprising distilling said isocyanate from saidmixture in the presence of about 25 to 300%, based on the weight of saidisocyanate, of a member of the group consisting of the lower alkyl andmonocyclic aryl triesters of orthophosphoric acid which are liquid underthe conditions of the distillation, said lower alkyl groups containing 1to 8 carbon atoms.

2. A process as defined in claim 1 wherein said distillation is carriedout at sub-atmospheric pressure.

3. A process as defined in claim 1 wherein said mixture is in the formof a-lsolution of the organic isocyanate in an inert organic liquiddiluent.

4. A process as defined in claim 1 wherein said organic isocyanate is anaromatic diisocyanate.

5. A process as defined in claim 1 wherein said organic isocyanate is2,4-tolylene diisocyanate.

6. A process as defined in claim 1 wherein said liquid triester ofortho-phosphoric acid is employed in proportions of about 50 to 100% ofthe weight of the organic isocyanate in said mixture.

7. A process as defined in claim 1 wherein said liquid triester oforthophosphoric acid is tri-o-cresyl phosphate.

8. A process for recovering an aromatic diisocyanate from a reactionmass produced by reaction of phosgene with an aromatic diamine in aninert organic liquid diluent to produce the corresponding aromaticdiisocyanate comprising adding to said reaction mass about 25 to 300%,based on the weight of said diisocyanate, of a member of the groupconsisting of lower alkyl and monocyclic aryl triesters oforthophosphoric acid which are liquid under the conditions of thesubsequent distillation, and then successively distilling said inertorganic liquid diluent and said aromatic diisocyanate from the reactionmass under vacuum, said lower alkyl groups containing 1 to 8 carbonatoms.

9. A process as defined in claim 8 wherein said aromatic amine is2,4-t0lylene diamine and said aromatic diisocyanate is 2,4-tolylenediisocyanate.

10. A process as defined in claim 9 wherein said liquid triester oforthophosphoric acid is tri-o-cresyl phosphate.

References Cited in the file of this patent UNITED STATES PATENTS2,680,128 Slocombe et a1. June 1, 1954 2,680,129 Flores June 1, 19542,680,130 Flores June 1, 1954 2,757,183 Irwin et al July 31, 19562,819,949 Keller et al I an. 14, 1958 FOREIGN PATENTS 761,592 GreatBritain Nov. 14, 1956 Patent No., 2,884 36l April 28 1959 Albert Bloomet ale It is hereby certified that error a of the above numbered patentrequiring 0 .Patent should read as corrected below.

ppears in the printed specification orrection and that the said LettersColumn 2, line 9 for "triesters or" read triesters of Signed and sealedthis 4th day of April 19610 (SEAL) Arre ERNEST W. SWIDER X XXXMXQMXARTHUR w. CROCKER Attesting Officer Acting Commissioner of Patents

1. A PROCESS FOR RECOVERING AN ORGAIC ISOCYANATE FROM A CRUDE MIXTURECONTAINING THE SAME, COMPRISING DISTILLING SAID ISOCYANATE FROM SAIDMIXTURE IN THE PRESENCE OF ABOUT 25 TO 300%, BASED ON THE WEIGHT OF SAIDISOCYANATE, OF A MEMBER OF THE GROUP CONSISTING OF THE LOWER ALKYL ANDMONOCYCLIC ARYL TRIESTERS OF ORTHOPHOSPHORIC ACID WHICH ARE LIQUID UNDERTHE CONDITIONS OF THE DISTILLATION, SAID LOWER ALKYL GROUPS CONTAINING 1TO 8 CARBON ATOMS.